During the combustion cycle of the engine, a piston reciprocates in known fashion within a cylinder which defines a combustion chamber into which an injection of fuel is provided to initiate the combustion process. The combustion chamber is closed, at its upper end, by a cylinder head which defines a so-called fire deck of the cylinder. Current diesel engines are likely to have four valves per cylinder; two intake valves and two exhaust valves. The valves normally take the form of mushroom-headed poppet valves which, when closed, seat against the fire deck of the cylinder and, when open, project into the cylinder. The valves are referred to as inward opening as they open into the combustion chamber. Such poppet valves are common place and their design parameters are well understood. However, it is one negative aspect of their operation that the exhaust valve is obliged to open against considerable cylinder pressure under certain full load running conditions, and also during engine braking operation. For this reason the valve train must be more robust than would otherwise be the case. Furthermore, by virtue of opening into the cylinder, the opportunity to provide charge scavenging during the intake/exhaust valve overlap period is severely restricted due to the probability of piston-to-valve collision unless clearance scallops are incorporated into the crown of the cylinder piston or the fire deck. However, these scallops have been found to degrade combustion through inhibition of desirable in-cylinder air swirl.
There have been many developments in diesel engine technology in recent years, in particular relating to improvements to the combustion process itself. Currently, the diesel engine combustion process can be described as heterogeneous and diffusion based, although the prevailing trend now is to move towards a premixed auto-ignition model of some type. The various advanced combustion modes may be characterised by, among other things, the lack of a positive initiator for the start of combustion, the requirement for relatively high levels of dilution of the charge with exhaust gas and the undesirable generation of high rates of cylinder pressure rise (dp/dt) resulting in excessive noise and structural stress. Under premixed combustion conditions, unacceptable values in the range of 10 to 35 bar/crank angle degree have been recorded for cylinder pressure rise rate, whereas a value of perhaps 5 bar/crank angle degree would be considered a normal maximum. Although advanced combustion modes provide benefits for efficiency and emissions, the aforementioned characteristics in particular present problems for which solutions are sought.
By way of example, U.S. Pat. No. 5,476,072 describes a cylinder head construction which is intended to address the stress induced by high rates of pressure rise in a valveless 2-stroke spark ignition engine.
Increasingly, fuel injection systems of the common rail type are specified for new diesel engines. In such systems a multi-piston pump raises fuel pressure to a level that is suitable for injection directly into the engine combustion chamber. Typically, the system includes a high pressure pump having a cam drive arrangement, an accumulator volume or rail, one injector per engine cylinder and an electronic control unit (ECU) for controlling injection timing and other parameters. The high pressure pump is a complex, heavy and costly component, and although common rail systems provide benefits over more traditional fuel injection systems (e.g. distributor pumps, unit pumps), there are significant implications on engine design with respect to the location of the high pressure pump and its drive arrangement. In particular, the drive torque signature can include “spikes” which make the pump and its drive a leading source of undesirable noise, vibration and harshness.
By way of background to the invention, the prior art in the following patent literature is acknowledged: U.S. Pat. Nos. 4,244,342 and 4,394,856 describe compression-pressure operated pumps combined with injector units, GB 465263 describes a compression-pressure operated pump for pressurising fuel for injection, and GB 590628 describes a fuel system where the pressure in one cylinder of the engine is used to pressurise lubricating oil for another cylinder of the engine.
It is an object of the invention to provide an improvement to known fuel systems which reduces or overcomes at least one of the aforementioned disadvantages associated with current systems. It is a further object of the invention to provide a novel exhaust valve arrangement for use in such fuel systems.